Cross reference is made to the following commonly assigned U.S. Patents to Eschbach et al., each of which is hereby expressly incorporated by reference herein: U.S. Pat. No. 5,414,538; U.S. Pat. No. 5,357,352; U.S. Pat. No. 5,450,502; U.S. Pat. No. 5,371,615; U.S. Pat. No. 5,363,209; U.S. Pat. No. 5,450,217; and, U.S. Pat. No. 5,802,214. Similarly, commonly assigned U.S. Pat. No. 5,347,374 to Fuss et al. is also hereby expressly incorporated by reference herein.
In the past, a typical application for copiers or scan-to-print image processing systems was to reproduce an input image as accurately as possible, i.e., render a copy. Thus, copies have been rendered as accurately as possible, including flaws in the source image. However, as customers have become more sophisticated in their document reproduction requirements, they have recognized that an exact copy is often not desired. Instead, an inexact copy that is perceived as a higher quality image has been deemed more desirable.
Until recently, image quality from the output of a copier or a scan-to-print system was directly related to the quality of the input image. While this has been satisfactory for reproducing images of text or line drawings, it has been found to be sub-optimal for purposes of reproducing photographs and other more complex images. With photographs, in particular, reproduction is complicated given the inexact nature of the discipline, variations in equipment, aging of photographs, and the like. Given that the input image is often poor quality, it has been deemed desirable to render an output image that is perceived to be the xe2x80x9cbestxe2x80x9d possible image (or at least superior to the input image), rather than an exact copy.
The above-noted patents to Eschbach et al. and Fuss et al. disclose an Automated Image Enhancement (AIE) system. This system receives an optionally sub-sampled description of the input image, and alters the tone reproduction curve (TRC)xe2x80x94a curve that defines the relationship of an input image signal to an output image signal for purposes of enhancementxe2x80x94and/or derives or alters a sharpness filter for that image on an image-by-image basis, as appropriate, so that the resulting output image is perceived to be superior to the input image. AIE is used, for example, to alter perceived exposure, luminance, sharpness, saturation, color balance, and the like to provide an output image that is preferable to the input image. It is important to note that AIE performs well even if it has to rely upon a sub-sampled image to statistically analyze the overall image. AIE does not require that each and every item of image information be analyzed. Of course, this sub-sampling speeds image enhancement operations and reduces the size/number of memory buffers required to implement the AIE system.
More particularly, for example, U.S. Pat. No. 5,414,538 entitled xe2x80x9cImage-Dependent Exposure Enhancementxe2x80x9d discloses a method of altering the perceived exposure of an output image produced from an input image that includes: (a) receiving the input image defined in terms of red-green-blue (RGB) signals; (b) converting the RGB signals to corresponding luminance-chrominance signals including at least one signal that represent overall image intensity; (c) comparing the intensity signal to upper and lower intensity threshold signals that define the acceptable levels of brightness and darkness in the image; (d) if one of the thresholds is exceeded, the image signal representative of image intensity is processed according to a select equation, and a TRC associated with the image is adjusted so that exposure characteristics of the resulting output image are perceived to be superior to those of the input image.
U.S. Pat. No. 5,450,502 entitled xe2x80x9cImage-Dependent Luminance Enhancementxe2x80x9d discloses a method of altering the perceived luminance of an output image produced from an input image that includes: (a) receiving the input image data defined in terms of a color space; (b) if required, converting the input image data into a luminance-chrominance color space wherein at least one term bears a relationship to overall intensity of the input image; (c) deriving a global intensity histogram for the overall input image; (d) filtering the histogram signal to flatten high peaks and low valleys without altering its relatively flat portions; and, (e) utilizing the filtered histogram signal to control TRC mapping in a device with which the image is to be rendered. In accordance with another aspect of the disclosed luminance enhancement method, the input image can be divided into plural regions, and a local intensity histogram signal can be derived for each region. If any of the local histogram signals are flatter than the global histogram signal, the local signals are summed and used in place of the global histogram as input to the histogram flattening filter.
The methods described in the Eschbach et al. ""538 and ""502 patents, and the other Eschbach et al. and Fuss et al. patents noted above, are described in connection with uncompressed image data. However, in many image processing operations, the image data is retrieved from an image storage device or other location or is otherwise supplied in a compressed form to minimize image storage space. Most commonly, the image data is compressed according to the Joint Photographic Expert Group (JPEG) recommendation ISO DIS 10918-1 that has become an international standard for lossy compression of still images.
Thus, heretofore, image enhancement operations according to the AIE systems described above have required that the JPEG (or otherwise) compressed images first be decompressed or xe2x80x9cdecodedxe2x80x9d for image enhancement operations. This is undesirable in that the decompression operation slows the overall enhancement operation, the image processing apparatus must be provided with additional memory to accommodate large amounts of uncompressed image data, and each JPEG or other lossy compression operation, itself, further degrades the image data due to data loss.
In light of the foregoing, it has been deemed desirable to provide a novel and non-obvious method for utilizing JPEG compressed image data in the image enhancement operations described in the above-noted Eschbach et al. and Fuss et al. AIE patents. This will allow JPEG compressed images to be enhanced for subsequent rendering without the image being decompressed, enhanced, and then recompressed.
In accordance with the present invention, a method and apparatus for image enhancement of JPEG compressed image data are provided.
In accordance with a first aspect of the present invention, a method of processing JPEG compressed image data comprises: (i) receiving JPEG compressed image data that represent an input digital image, the JPEG compressed image data including a plurality of DC components; (ii) extracting a plurality of the DC components from the JPEG compressed image data; (iii) inputting at least some of the extracted DC components to an automated image enhancement system; (iv) using the DC components input to the automated image enhancement system to derive a final correction tone reproduction curve and/or a sharpness filter for enhancement of the input digital image represented by the JPEG compressed data; (v) decompressing the JPEG compressed image data to obtain decompressed image data that represent the input digital image; and, (vi) using the final tone reproduction curve and/or the sharpness filter to enhance the decompressed image data.
In accordance with another aspect of the present invention, an apparatus for processing JPEG compressed image data is provided. The apparatus includes means for receiving JPEG compressed image data that represent an input digital image; a DC component extractor adapted for extracting a plurality of the DC components from the JPEG compressed image data; an automated image enhancement system adapted for receiving some or all of the extracted DC components and for using same to derive a final correction tone reproduction curve and/or a sharpness filter for enhancement of the input digital image represented by said JPEG compressed data; means for decompressing the JPEG compressed data to provide uncompressed image data that represent the input digital image; and, means for using the final tone reproduction curve and/or the sharpness filter to enhance the uncompressed image data provided by the decompressing means.
In accordance with still another aspect of the present invention, a method of subsampling JPEG compressed image data in an automated image enhancement system includes: (i) receiving blocks of JPEG compressed image data, wherein each of the blocks represents a plurality of pixels of an input digital image and includes a DC component; (ii) without altering the JPEG compressed data, extracting the DC components from at least some of the blocks of JPEG compressed data; (iii) inputting at least some of the extracted DC components to an automated image enhancement system; and, (iv) using the DC components input to the automated image enhancement system to generate a correction tone reproduction curve and/or a sharpness filter that are to be applied to the input digital image represented by the JPEG compressed data after the JPEG compressed data are decompressed.
One advantage of the present invention resides in the provision of a method and apparatus for enhancing JPEG compressed image data, without fully decompressing the data.
Another advantage of the present invention is found in the provision of a method for enhancing JPEG compressed data in terms of exposure, luminance, sharpness, and otherwise utilizing only the DC components of JPEG compressed data.
Still another advantage of the present invention is the provision of a method for enhancing JPEG compressed image data wherein use of the JPEG DC components only for image enhancement operations necessarily sub-samples the image data according to the JPEG data blocks so that the image is sub-sampled by a factor of at least 64 for black-and-white data and by a factor of at least 192 for color data.
A further advantage of the present invention is the provision of a method for enhancing JPEG compressed image data that minimizes image data loss due to repetitive JPEG compression and decompression operations.
Still another advantage of the present invention resides in the provision of a method for enhancing JPEG compressed image data that is usable in image processing apparatus that are not able to store a full decompressed digital image due to memory constraints.
Yet another advantage of the present invention is found in the provision of a method for enhancing JPEG compressed digital images that have been scanned previously without image enhancement processing and stored in JPEG compressed in a data base.
A still further advantage of the present invention resides in the provision of a method and apparatus for enhancing JPEG compressed digital images wherein the enhancement TRC and/or filter can be bound to the JPEG compressed data for subsequent use when decompression of the JPEG compressed data is otherwise required.
Still other benefits and advantages of the present invention will become apparent to those of ordinary skill in the art to which the invention pertains upon their reading and understanding this specification.